The invention relates to a semiconductor laser with a type I laser structure which achieves improved efficiency and greater maximum wavelengths.
A plurality of atmospheric transmission windows and absorption bands of numerous gases and chemical substances are situated in the optical spectral range between 2 and 4 μm (partial range of the intermediate infrared). Furthermore, water has its absolute absorption maximum at a wavelength of 3 μm. Efficient and compact laser sources in this wavelength range are hence of central importance for a large number of applications, both in scientific, medical and commercial and in safety-relevant and military spheres.
Conventional types of laser, such as e.g. thulium-, holmium- or erbium-doped solid or fibre lasers, in fact emit in this wavelength range, however only isolated emission lines or bands of the different laser materials exist here, which do not enable complete coverage of the mentioned spectral range. Hence, with these types of laser, adaptation of the emission wavelength to concrete applications can be effected only in a restricted manner.
However, adaptation can be achieved by using semiconductor lasers based on III/V compound semiconductors. By varying the layer structures and material compositions, in principle emission wavelengths which completely cover the intermediate infrared can be achieved here. Thus, in the intermediate infrared, several types of such semiconductor lasers have existed to date, a differentiation being able to be made here between electrically- and optically-pumped lasers.
Examples of electrically-pumped semiconductor lasers in the intermediate infrared are double heterostructure diode lasers, GaSb-based “type I” lasers with emission wavelengths between 1.8 and 3.3 μm and also quantum cascade lasers (QCL) with λ≧3 μm. Optically-pumped lasers in this spectral range are e.g. lasers with “type II” or “W” laser structure (wavelength λ=2.4-9.3 μm).
Each of these concepts has advantages and disadvantages specific to the type of construction. By way of example, QCLs emit a strongly divergent output beam and achieve in particular, in the wavelength range relevant for this invention (λ≦4 μm), only low efficiency, output powers and maximum operating temperatures (J. Devenson, R. Teissier, O. Cathabard and A. Baranov, “InAs/AlSb quantum cascade laser emitting below 3 μm”, Appl. Phys. Lett. 90, 111118 (2007)). Furthermore, surface-emitting QCLs cannot be produced because of basic physical effects. With InAs/GaSb-based lasers with type II laser structure, somewhat shorter wavelengths are achieved with higher efficiency [R. Kaspi, A. Ongstad, G. Dente, J. Chavez, M. Tilton and D. Gianardi, “High performance optically pumped antimonide lasers operating in the 2.4-9.3 μm wavelength range”, Appl. Phys. Lett. 88, p. 041122 (2006)], however, because of a spatially indirect electronic transition between conduction- and valency band, the optical gain required for the laser activity, in comparison with type I lasers with spatially direct transmission, is lower. The two proposed types of laser are dependent for efficient laser operation, in particular for the continuous wave mode, upon (in part cryogenic) cooling, which can greatly restrict the field of application.
In semiconductor lasers with “type I” structure of the active region, laser radiation is produced in that, in the active quantum films or quantum wells (QW) in the valency band (VB) and typically in the heavy hole band (HH band), electron hole pairs are recombined with stimulated emission of a laser photon. The electron hole pairs are produced by pumping in the barrier layers, then migrate into the quantum film and recombine there. In GaSb-based type I lasers with emission wavelength above approx. 1.7 μm, the quantum films with a thickness of a few nanometers generally consist of GaxIn1-xSb or GaxIn1-xAsySb1-y, x and y respectively describing the molar proportion of Ga (gallium) or As (arsenic) in the quantum film material. The quantum films are normally embedded in AlzGa1-ZAsySb1-y barrier layers, z representing the molar proportion of Al (aluminium) in the quaternary barrier material. The arsenic content y of the barrier layers is typically adjusted such that these can be grown lattice-adapted to the GaSb substrate. The lattice adaptation of the barrier layers is however not a compulsory condition for production of a semiconductor laser structure. In general, barrier layers with a molar aluminium proportion z of approx. 30 to 35% are used.
During the laser operation, the temperature in the active region of the lasers can be above the heat sink temperature by some ten degrees. The efficiency of GaSb-based type I lasers is hereby greatly reduced, in particular at wavelengths above approx. 2.4 μm.
Furthermore, in the case of lasers in which a plurality of quantum wells are placed one behind the other, a non-homogeneous occupation of the quantum wells with electrons results, which likewise can impair the efficiency of the laser.